The speech synthesizer is used to convert digitized sound information to analog signal and output to a speaker to regenerate the sound. FIG. 1A shows a waveform of a period of sound and the digitized range of the wave. When digitally encoded with 7-bits pulse code modulation (PCM), it normally uses 40H as the central point, and the speech signal varies up and down around the central point, that is varies in-between 00H and 7FH.
FIG. 2 shows a functional block diagram of a conventional PCM speech synthesizer connected with a speaker, in which the PCM codes are stored in the PCM encode register 22. When the speech synthesizer 20 is enabled and start to operation, the PCM codes that stored in the PCM encode register 22 are sent out to the digital to analog converter unit 24 in order. The digital to analog converter unit 24 converts the received PCM codes to an analog signal and then sends to the output terminal VO. The current from the output terminal VO is amplified by a transistor 26 to drive the speaker 50 to produce sounds. To save the power during the speech synthesizer 20 is turned off, there shouldn't be any current from the output terminal VO, therefore the initial value of the PCM encode register 22 is 00H. As shown in FIG. 1B, when the speech synthesizer 20 is enabled, PCM codes are sent to the digital to analog converter unit 24, and the value changed from 00H to 40H, that makes an large instant current change on the output terminal VO and thus produces burst noises. On the other hand, when the operation of the speech synthesizer 20 stops, PCM code changes from 40H to 00H, that also makes an large instant current change on the output terminal VO and produces burst noises. For improvement, it always adds a rising wave and a falling wave before and after the sound data respectively to avoid the large instant current change and burst noises, as shown in FIG. 1C.
Another type of speech synthesizer that drives the speaker directly, for example the push-pull drive type, is shown in FIG. 3, which processes with pulse width modulation (PWM). The PWM speech synthesizer 30 includes a PWM encode register 32 and a digital to analog converter unit 34. The digital to analog converter unit 34 has two output pins and sends out signal with push-pull type to drive the speaker 50 directly. The speech synthesizer 30 uses 40H as the zero point of the speech signal, thereby the initial value is 40H before the speech synthesizer 30 is enabled. In such circumstances, both the initial value and the central point of the speech signal are 40H, it is therefore not necessary to add a rising wave and a falling wave before and after the sound data.
However, when it is intended to integrate the PCM type with the direct drive type speech synthesizers, it is necessary to provide a mechanism to decide which type of speech synthesis unit to be enabled since the speaker connection and the initial value are different for them. Typically, there are two methods to provide needed information. The first one is to provide an input pin for selection. However it needs more chip area, and it requests the user to control the state of the input pin. When the input pin is not under good control, the speech synthesizer can not function correctly, thus it is inconvenient for applications. The second method is to select by mask (in the encoding layer or metal layer). Once the chip is fabricated, it is impossible to change the speaker connection and thus has less flexibility.